Alkali metal chlorides in the form of natural or artificial salts and brines have widespread use in many industrial processes. In particular, the electrochemical production of chlorine, alkali metal hydroxide, and related products makes extensive use of alkali metal chloride brine. Nearly all commercial salts and brines are contaminated to some degree by various impurities, and while the level at which such impurities may be tolerated is dependent upon the specific process and method of operation, it is desirable in electrochemical uses to obtain a brine of comparatively high purity.
The presence of metallic impurities is of considerable concern in the electrolysis of brine to produce chlorine and alkali metal hydroxide, since trace metals encourage the evolution of hydrogen in the electrochemical cell. Such hydrogen generation is undesirable, as the combination of hydrogen, chlorine and oxygen forms an explosive mixture over a wide range of proportions. In practice it is preferred to limit the amount of hydrogen present in the chlorine to less than one percent. This is especially critical where the chlorine product is to be liquefied or absorbed, since the proportion of hydrogen in the remaining gas may quickly rise into the explosive range.
The effect of metallic impurities on hydrogen gas evolution in chlor-alkali cells has been extensively studied, and it has been determined that heavy metals such as vanadium, antimony, molybdenum and arsenic have a considerable catalytic effect on hydrogen formation in the cell. Many other metals such as aluminum, calcium, and magnesium also promote hydrogen evolution, and it has been found that combinations of two or more metals often have more effect than do the same metals taken separately, e.g. magnesium and iron form a synergistic pair. It has thus been the practice to reduce metallic impurities in electrolytic brine to the lowest possible level.
In the conventional purification processes, calcium has been removed from brine by the addition of alkali metal carbonate and the resultant precipitation of calcium carbonate. Iron and magnesium impurities have been removed by precipitation as the hydroxides, usually by the addition of alkali metal hydroxide. The sulfate radical is generally removed by the addition of a barium salt such as barium carbonate or barium chloride, which brings about the precipitation of barium sulfate. In these precipitation processes a coagulation, settling or filtering operation is used to rid the brine of the precipitated impurities prior to use.
However, these conventional purification techniques often fail to reduce the level of metals such as aluminum and heavy metals such as antimony, arsenic, molybdenum, and vanadium to the degree necessary for satisfactory use of the brine in electrochemical cells. It would therefore be desirable to provide a straightforward, efficient method for the removal of trace metal impurities from brine.